This invention relates to a diagnostic x-ray imaging system. It relates more particularly to an improved large area radiation detector for use in such a system.
In diagnostic x-ray applications, a patient is illuminated by a beam of x-rays or gamma rays. Upon passing through the patient's body, some of the photons in the beam are absorbed, others are not, depending upon the densities of the tissues, bones, fluids, etc. along the tracks followed by the photons through the body. Thus the radiation pattern emerging from the patient's body can be used to produce an image indicating the composition of the body along the path of the radiation beam. This image can be reduced to visible form by exposing a film to the emergent radiation or by exposing an array of photo detectors which develop electrical signals corresponding to the emergent radiation to produce an electronic display of the body image.
The photosensitive coating on the film is quite thin and the photosensitive grains themselves are not very dense. Consequently only a relatively small percentage of the incident photons interact with the film to produce the resultant image. These factors also contribute to the relatively large radiation dosage required to produce an acceptable picture in prior apparatus.
This problem is somewhat alleviated through the use of film-screen combinations in which an intensifying screen which converts x-rays to light is placed in contact with the photosensitive film. Since the screen absorbs a greater fraction of x-rays than the film, the quantum detection efficiency is raised and the required radiation dose is lowered. However, this device suffers from the following drawback. It cannot be known that a particular image is satisfactory until after the film has been developed. This takes an appreciable amount of time during which the patient must remain available in case additional pictures are required. Needless to say, this is annoying to the patient. Also, it slows down the processing of patients through the radiology laboratory. Furthermore, if the image is unsatisfactory, the exposure may have to be repeated, thus increasing the radiation dose to the patient.
The prior systems that produce radiation images electronically have tended to be relatively complex and expensive and limited in sensitive area.
Recently there has been developed a radiation detector employing a so-called superheated, superconducting colloid (SSC). Basically, the detector comprises a colloid body composed of small grains of a dense superconducting material suspended in a less dense binder. The body is subjected to a low temperature and an external magnetic field that maintains the grains in a metastable superconducting state in the absence of radiation. When a grain is impacted by a photon, it undergoes a transition from the metastable superconducting state to the normal conducting state. This transition produces a magnetic flux change in the region of the grain and the flux change is detected by a sensing coil on the surface of the body that has at least one loop encircling the grain in question. As each grain within that loop changes to its normal conducting state in response to an incident photon, a voltage pulse is developed in the sensing coil reflecting that change. Consequently the number of pulses detected provides an indication of the intensity of the radiation incident on the detector. A detector such as this is disclosed, for example, in French Pat. Nos. 7536494 and 7536495.
Until now, however, the so-called SSC detectors have had relatively low quantum detection efficiency and small surface area. Therefore, they have not been used in nuclear medicine or radiology applications. These problems stem from the fact that the usual SSC detector is relatively thin (e.g. 1 mm). Therefore a relatively high percentage of the incident photons do not have an opportunity to interact with, and be absorbed by, the colloid grains in the detector. It is no solution to increase the thickness of the usual SCC detector because a sensing coil in the detector having a width W which defines the spatial resolution responds to flux changes produced only by those grains which reside within a distance of approximately W/2 above or below the plane of the coil. Therefore a sensing coil on the surface of the typical colloid body does not detect photon interactions with grains near the center of the body. Likewise, a coil imbedded in the body may not detect events occurring near the surfaces of the body. Thus even though more photons may interact with the detector grains, many of these interactions would not be detected by the sensing coil so that there would be no net gain in quantum detection efficiency or resolution. Thus, in order to obtain an image of the object being irradiated with the prior SSC detectors, a relatively long period of exposure would still be required, presenting a potential health hazard in the case of animals and humans.